The Celestia WikiBook contains technical documentation on Celestia for users, add-on creators, and developers. Experienced Celestia users are invited contribute to the Wikibook themselves.
Please see the documentation page at The Celestia Motherlode for a comprehensive list of documentation available for Celestia. In addition to documentation for Celestia users, there are many files of interest to people who want extend Celestia by creating add-ons and scripts.